This invention relates to a control unit for use in an injection molding machine and, in particular, to a control unit of the type which is capable of improving a response characteristic and a tracking property in a pressure control.
Recently, wide use has been made of an electrical injection molding machine which is driven by a servo motor in place of a hydraulic actuator to inject a resin material into a mold and to manufacture a molded product. In general, the injection molding machine includes an injection unit driven by the servo motor and a controller coupled to the injection unit and the servo motor to control the servo motor by monitoring the injection unit. The injection unit comprises an injection cylinder provided with a hopper and a screw arranged in the injection cylinder. The screw is directly coupled at its rear end to a drive shaft operatively coupled to an injection motor formed by the servo motor. The servo motor rotates the drive shaft to move the drive shaft forwards and backwards. As a result, the screw reciprocates forwards and backwards along a guide bar. A load cell is mechanically coupled to the drive shaft to detect a pressure imposed on the drive shaft.
Description will be made as regards an operation of the injection unit hereinafter.
(1) A resin material is introduced from the hopper into the injection cylinder and is melted into molten resin. The molten resin is conveyed towards a top end portion of the heat cylinder by a predetermined amount by driving the injection motor. This process is referred to as a measuring process. During the measuring process, the screw is slightly retracted due to a back pressure of the molten resin packed into the top end portion of the injection cylinder. The back pressure is detected by the load cell which produces a pressure detection signal representative of the back pressure. The back pressure is controlled by the use of the pressure detection signal in a manner to be described later.
(2) Then, the drive shaft is driven by the injection motor to be moved forwards. A forward end of the screw acts as a piston to inject the molten resin into the mold. This process is referred to as a filling or an injection process. During the injection process, the controller puts the screw into a velocity or speed control mode.
(3) At the end of the injection process, the mold is filled with the molten resin. Then, the controller switches a control mode from the velocity control mode into a pressure control mode. Such a switching operation is called a V-P (velocity to pressure) switching operation. The timing and the manner of the switching operation seriously affect the quality of a molded product.
(4) After the V-P switching operation, the molten resin filled in the mold is cooled and solidified under a predetermined pressure. This process is referred to as a holding process. The pressure applied to the resin is controlled by the controller, like the back pressure.
In the injection unit, the holding process (4) is further succeeded by a next measuring process (1) in a next cycle.
On the other hand, a clamping unit is operated to clamp or close the mold during the holding process (4) which is followed by an ejecting process. In the ejecting process, a molded product which has been cooled and solidified is removed or ejected from the mold by opening the mold and by using an ejector mechanism. The mold is thereafter closed again and the ejecting process proceeds to the injection process (2). Thus, the measuring process (1) in the injection unit is carried out simultaneously with the ejecting process in the clamping unit.
During the injection process, a velocity feedback control is carried out by the use of the movement or velocity of the screw. On the other hand, during the holding process, a pressure feedback control is carried out by detecting the reaction force applied to the load cell. Thus, the molding operation is carried out by a combination of the velocity feedback control and the pressure feedback control.
In a conventional pressure control system, an operation pattern signal representative of a target value is produced by an operation pattern generator. For example, such an operation pattern generator is disclosed in Japanese Patent Prepublication No. 253319-1991. The reaction force applied to the load cell is detected as a pressure detection signal. The pressure detection signal is amplified and then subtracted from the operation pattern signal to produce a first error signal. The error signal is multiplied by a basic gain to produce a multiplication result signal. The multiplication result signal is subjected to a compensation calculation to produce a compensated result signal. On the other hand, a velocity detection signal is obtained by amplifying and differentiating a position signal. The velocity detection signal is subtracted from the compensated result signal to produce a second error signal. The second error signal is subjected to a compensation calculation and amplified into a torque command signal.
In the conventional pressure control system described above, an operation characteristic in the pressure control is determined with reference to a single basic gain that is preliminarily selected. With this structure, it is difficult to uniquely define the load in the pressure control system because a wide variety of resin materials and molds are used. In this connection, the gain must be selected at a minimum value in consideration of various conditions. Otherwise, objectionable oscillation occurs in the pressure control system.
When the gain is increased to improve the response characteristic of the pressure control system, a transmission mechanism from the servo motor to the screw may be oscillated due to the remarkable increase in backlash or a difference in efficiency between forward and backward movement of the screw. Under these circumstances, the gain is usually selected to be considerably small. This results in deterioration of accuracy and repeatability of the pressure control.